Abstract: Small RNA mediated regulation of gene expression is a recent addition to fundamental gene regulatory mechanisms that contribute to coordinate temporal and spatial gene expression of possibly every gene of a eukaryotic genome. The predominant sources of small RNA in somatic tissues are miRNA genes that encode short dsRNA hairpins of evolutionary conserved sequences. Disorders of the nervous system, such as schizophrenia, depression, bipolar disorder or autism are poorly understood at a molecular level but presumed to be caused by perturbation in gene regulatory networks of neuronal circuits. In this application we propose to explore if miRNA regulatory networks play a role in these and other diseases of the nervous system. This will be achieved through a highly collaborative and interactive effort of three laboratories that will develop methods for identification of small RNAs and their regulated targets and use biochemical, cell and animal model systems to study the detailed molecular mechanisms of gene regulation. The specific aims of this study are: 1. Define the small RNA content of anatomical and/or functionally distinct brain regions of normal and disease state. Include in the analysis model neuronal and nervous system cell lines as well as neuronal stem cell progenitors and derived cells or structures. 2. Define the spatial and cell type specific expression of the most abundantly and specifically expressed miRNAs in the brain of mouse, Macaca fascicularis, and human brain (normal and disease states). 3. Define the targets of miRNAs in mouse brain and normal and patient brain regions using genomic and biochemical approaches. 4. Determine the role of miRNAs in neuronal differentiation of mouse embryonic stem cells. Lay statement: Although efforts to identify genes responsible for major depression, bipolar disorder, autism, and schizophrenia have been challenging in the past, the purpose of this study will be to provide an expression atlas of a new class of small non-coding genes (microRNAs) from normal and mentally disabled subjects to determine the role of a subset of the highly expressed microRNAs in the central nervous system. Understanding the specific interactions of microRNAs and the network of genes they target and regulate may ultimately contribute to our understanding and treatment of these debilitating mental illnesses.